This invention relates to the field of diffusion bondable structures, superplastic forming techniques, and more particularly to structures made from monolithic metal or metal matrix composite materials.
Performance requirement goals for the National Aerospace Plane (NASP), which is a manned hypersonic transport vehicle, require airframe structures and engine materials that exceed the capabilities of commercially available materials and manufacturing technologies. However, for some time experimental work has been underway with metal-matrix composites which are similar to the well known polymeric composites except that the matrix is a metal rather than a polymer in an effort to produce higher temperature and lower weight space structures. Metal-matrix composites are fabricated by placing a reinforcing material such as silicon carbide fibers between foils of a matrix material such as a metal alloy. These ingredients are then consolidated into a composite by pressing them together at high temperatures and pressures which cause the matrix to flow around the reinforcing fibers and diffusion bond the matrix together to form a unique composite. Different alloys of aluminum, magnesium, and titanium have been used as the matrix materials and as the reinforcing phase fibers, powders, and whiskers made from silicon carbide, alumina, graphite, boron and other materials have been used. The actual material used to prove the process and tools discussed herein was titanium 15-3-3-3 with reinforcing fibers of silicon carbide. However the process was developed first using monolithic titanium alloy 15-3-3-3.
The elements, when using the monolithic titanium, may be formed superplastically or by any other conventional method. FIG. 4 shows a section through a typical large airplane wing spar with the flanges canted with respect to the web. These spars are currently machined on a spar mill and due to long spar length, amount of material to be removed, and post heat treatment are very expensive to fabricate.
Although several different methods for fabricating the metal matrix composite elements have been used, the most successful current procedure uses a hot isostatic press (HIP). These chambers achieve diffusion bonding temperatures while applying pressures upwards of 15,000 PSI. Obviously, these chambers are not only in short supply but their use is expensive. Either sheet material or simple structural sections like channel sections can be produced in the HIP chamber. In order to obtain a sound composite element with optimum mechanical properties it is necessary to consolidate the matrix with the reinforcement phase by applying extreme pressures to avoid poor interfacial bonding between the graphite and the metal matrix. It is well known in the diffusion bonding art that the bond improves substantially with increased pressure, up to a point.
Simple sections, like the channel sections, are formed in the HIP chamber just as the sheet stock is by applying alternate layers of metal foil and reinforcing fibers or cloth. In the case of the simple sections they are formed in the shape of the section rather than a flat sheet. In order to form more complex shapes, at least by the present known art, you would have to combine preformed simple shapes in order to form the complex shapes so that the result is that it is at least a two step operation in the HIP chamber. Further, in the HIP chamber, in order to maintain an inert environment around the specimen to avoid oxidizing, it is necessary to maintain the specimen in a bag and there is no simple way to monitor leaks in the bag so that if a leak develops in the bag the whole shot is lost.
It is an object of the present invention to provide a method and tools to fabricate complex structural shapes made from either metal matrix composite or monolithic metal elements e.g. I-beams, tapered I-beams, I-beams with a sinusoidal shaped web section, and conventional cross-section beams e.g. T, X, Y, H, L and Z sections used in aircraft structures.
It is a further object of this invention to provide the above-noted complex structural shapes, in the case of the metal composites, using the HIP chamber only in the initial phase. This phase includes consolidating the materials into the composite elements which may include the formation of simple shapes, which, in this invention, are combined to make the complex shapes.
Yet a further object of this invention is to produce complex structures from both diffusion bondable monolithic metal or metal matrix composites and non-fusion bondable monolithic metal or metal matrix composites by the introduction of suitable brazing foil between the elements.
As a final object of this invention, it is intended to provide a process which can be monitored so that the metal vacuum bag surrounding the specimen at the time of formation and the pressurizing bladders may be monitored for leaks; in the event a leak occurs, the process may be stopped and the leak corrected without losing the specimen.